1. Field of the Invention
The present invention relates to platinum powders and to methods for producing such powders, as well as intermediate products and devices incorporating the powders. In particular, the present invention is directed to powder batches of platinum metal particles with a small average particle size, well controlled particle size distribution, spherical morphology and high crystallinity.
2. Description of Related Art
Many product applications require metal-containing powders with one or more of the following properties: high purity; high crystallinity; small average particle size; narrow particle size distribution; spherical particle morphology; controlled surface chemistry; reduced agglomeration of particles; and high density (low porosity). Examples of metal powders requiring such characteristics include, but are not limited to, those useful in microelectronic applications, such as for conductive paths interconnecting discrete components in multi-chip modules, or the like. Platinum metal has particular applicability for applications requiring high reliability.
Platinum metal powders are used in hybrid microelectronic components where high reliability and high performance over an extended length of time are critical. Such uses include military applications, medical devices and aerospace applications. Typically, the platinum metal powder is dispersed into a paste which is screen printed onto a circuit board and sintered to produce a conductive line. Platinum metal has a higher reliability over extended periods of time than, for example, base metals. Platinum is also malleable and ductile. Platinum does not substantially oxidize and therefore can be heated in air to temperatures up to its melting point. Platinum is also resistant to corrosion in a variety of conditions.
Various methods have been suggested for producing platinum metal powders. For example, U.S. Pat. No. 3,620,713 by Short discloses a method for making a platinum metal powder for electronic components having an average particle size of from 0.5 to 2 .mu.m. A platinum metal chloride solution is formed, platinum is precipitated from the solution as an ammonia complex by adding ammonium hydroxide and the complex is reduced with a reducing agent to produce platinum metal. U.S. Pat. No. 3,620,714 by Short discloses a similar process for producing alloys of platinum and another noble metal.
It has also been suggested to form particles of platinum with a refractory oxide such as alumina and silica. For example, U.S. Pat. No. 4,274,877 by Collier et al. discloses a method for forming metal composite particles having the average size of 0.2 .mu.m to 2 .mu.m. It is disclosed that the particles include about 10 to 25 weight percent of a refractory material. The particles are formed by activating the refractory material, nucleating metal particles on the surface of the refractory material and growing the metal on the refractory surface.
Despite the foregoing, there remains a need for platinum-based metal powders having a combination of improved properties such as a small particle size, narrow particle size distribution, high crystallinity (large crystals) and spherical morphology. It would be particularly advantageous if such platinum metal powders could be produced in large quantities on a substantially continuous basis.